Galactose, a monosaccharide, plays diverse biological roles in energy production, especially in the glycolysis and glycosylation of proteins and lipids. Galactose metabolism is mediated by the Leloir pathway, which comprises four key enzymes. Following lactose hydrolysis, galactose mutarotase (GALM) catalyzes the anomerization of β-D-galactose to α-D-galactose, providing a substrate for the downstream pathway. In 2019, GALM deficiency was defined as the fourth type of galactosemia. Affected individuals may develop cataracts similar to those observed in individuals with galactokinase deficiency, disrupting the subsequent steps in the Leloir pathway. However, cataracts generally occur less frequently and tend to be milder in patients with GALM deficiency, likely because of the partial compensation provided by spontaneous galactose mutarotation in aqueous solutions. Because lactose, the primary dietary source of galactose, is the predominant carbohydrate consumed until weaning, the timely initiation of lactose restriction can prevent or even reverse cataract formation. To date, other complications or adverse events, including those in heterozygous carriers of GALM variants, have not been clearly demonstrated. This review aims to synthesize current knowledge and findings of GALM deficiency on molecular mechanisms, clinical presentation, diagnostic approaches, carrier risk, and dietary management, with particular emphasis on cataract prevention and reversibility through early lactose restriction. By consolidating available evidence, we propose future research directions, with broader implications for newborn screening programs, clinical decision-making, and a deeper understanding of galactose metabolism.

Galactose mutarotase (GALM) deficiency was first reported in 2019 as the fourth type of galactosemia. This study aimed to investigate the clinical and genotypic spectra of GALM deficiency. This was a questionnaire-based retrospective survey conducted in Japan between February 2022 and March 2023. We identified 40 patients with GALM deficiency in Japan (estimated prevalence: 1:181,835). Four of 38 patients (10.5%) developed cataracts, which resolved with lactose restriction in 3 out of 4 patients. Transient transaminitis was the most common symptom (23.1%). All of the patients followed lactose restriction; discontinuation of the restriction after infancy did not cause any complications. Moreover, none of the participants experienced long-term complications. Two variants, GALM NM_138801.3: c.294del and c.424G>A, accounted for 72.5% of the identified pathogenic variants. The patients showed moderately elevated blood galactose levels with lactose intake; however, the elevation was lower than that observed in galactokinase deficiency. GALM deficiency is characterized by a similar but milder phenotype and lower blood galactose elevation than in galactokinase deficiency. Diagnosis and initiation of lactose restriction in early infancy should be essential for prevention of cataracts, especially in cases of irreversible opacity. Galactokinase deficiency, aka galactosemia type II, is an inborn error of galactose metabolism. Galactokinase deficiency is rare and more insidious than other galactosemia types since it results in the formation of nuclear cataracts without provoking intolerance symptoms or other systemic symptoms.

Congenital cataract can be caused by several systemic diseases and differential diagnosis should be done between infections, genetic or metabolic diseases. We present a case of a 12-month-old girl with bilateral nuclear cataracts that was referred for investigation. Since she did not present a family history of congenital cataracts or metabolic diseases, and her physical examination was normal, a systemic evaluation was performed. Biochemical studies disclosed abnormal galactose metabolism signs. The diagnosis of galactokinase (GALK1) deficiency was considered and the study of the GALK1 gene allowed identifying a pathogenic genetic variant and a predictably pathogenic missense mutation, previously not described. Dietary measures were imposed with a good evolution.

Galactokinase (GALK1) deficiency is a rare hereditary galactose metabolism disorder. Beyond cataract, the phenotypic spectrum is questionable. Data from affected patients included in the Galactosemias Network registry were collected to better characterize the phenotype. Observational study collecting medical data of 53 not previously reported GALK1 deficient patients from 17 centers in 11 countries from December 2014 to April 2020. Neonatal or childhood cataract was reported in 15 and 4 patients respectively. The occurrence of neonatal hypoglycemia and infection were comparable with the general population, whereas bleeding diathesis (8.1% versus 2.17-5.9%) and encephalopathy (3.9% versus 0.3%) were reported more often. Elevated transaminases were seen in 25.5%. Cognitive delay was reported in 5 patients. Urinary galactitol was elevated in all patients at diagnosis; five showed unexpected Gal-1-P increase. Most patients showed enzyme activities ≤1%. Eleven different genotypes were described, including six unpublished variants. The majority was homozygous for NM_000154.1:c.82C>A (p.Pro28Thr). Thirty-five patients were diagnosed following newborn screening, which was clearly beneficial. The phenotype of GALK1 deficiency may include neonatal elevation of transaminases, bleeding diathesis, and encephalopathy in addition to cataract. Potential complications beyond the neonatal period are not systematically surveyed and a better delineation is needed.

Galactokinase deficiency is an inborn error of carbohydrate metabolism due to a block in the formation of galactose-1-phosphate from galactose. Although the association of galactokinase deficiency with formation of cataracts is well established, the extent of the clinical phenotype is still under investigation. We describe a 6-year-old female who was diagnosed with galactokinase deficiency due to cataract formation when she was 10 months of age and initially started on galactose-restricted diet at that time for 5 months. She developed gait abnormality at 4 years of age. Breath tests via measurement of 13C isotope in exhaled carbon dioxide following 13C-labeled galactose administration at carbon-1 and carbon-2 positions revealed oxidation rates within the normal range. The results in this patient strikingly contrast with the results of another patient with GALK1 deficiency that underwent breath testing with [1-14C]-galactose and [2-14C]-galactose. Extension of in vivo breath tests to other galactokinase patients is needed to better understand the pathophysiology of this disease.